Process for production of glass preform for optical fiber

Abstract: In order to provide an improved process for the production of a glass preform for an optical fiber which includes substantially no bubble therein and also have a substantially uniform shape, there is provided a process for the production of a glass preform which comprises at least silica for an optical fiber comprising producing a body comprising a rod on which fine glass particles are deposited preferably by the vapor phase reactions and heating the body to vitrify under a reduced atmosphere or a vacuumed atmosphere so that the glass preform is produced; in which process the heating comprises a first heating step of degassing the body at a first heating temperature, a second heating step of shrinking the body at a second heating temperature which is higher than the first heating temperature and which is lower than a third heating temperature, and a third heating step of vitrifying the shrunken body at the third temperature which corresponds to a vitrification temperature of the fine glass particles so as to produce the glass preform.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a process for the production of a glass preform for an optical fiber, and in particular a glass preform comprising at least silica from which an optical fiber is directly drawn without any additional treatment for the preform or an intermediate glass preform from which an optical fiber is drawn after some additional treatment for the preform. Description of the Related Art A glass preform comprising at least silica for an optical fiber is produced by heating, at an elevated temperature, a body comprising a rod on which fine glass particles are deposited by the vapor phase reactions (which deposition process is known in the field as, for example, the outside chemical vapor deposition process and the vapor phase axial deposition (VAD) process) in an electrical furnace so that the glass particle deposited body is vitrified (namely consolidated). Such a vitrification process has been conventionally carried out by heating the body at an atmospheric pressure under an atmosphere filled with helium gas or an inert gas containing a small amount of a halogen gas. In such a process, a gas contained in the fine glass particles of the body remains confined within the body when the body is vitrified, which causes a problem that the vitrified body (namely the vitrified preform) includes bubbles therein. In order to solve such a problem, Japanese Patent Kokai Publication No. 63-21025 discloses a process for the vitrification of the glass preform in which the preform is heated under a vacuumed atmosphere or a reduced atmosphere. In this process, since the atmosphere is reduced or vacuumed, the body comprising the fine particles are degassed so that it is expected that no bubble is left in the vitrified body. Fig. 2 schematically shows a conventional apparatus for the production of the glass preform for the optical fiber. The apparatus comprises a vacuum vessel (10) in which a muffle tube (2) surrounding a fine particle deposited body (1) and a heating member (8) positioned outside the muffle tube (2) are placed and a thermal insulation (a heat shield) (9) is located between the heating element 8 and the wall of the vessel (10). The vessel (10) is further equipped with an evacuation port to which a vacuum pump (7) is connected through a vacuum line (6) so that the atmosphere inside the vessel can be vacuumed or reduced. The glass particle deposited body (1) inserted in the muffle tube (2) is vitrified there by heating it with the heating member (8). However, it has been found that the bubbles remain in the preform when the apparatus as shown in Fig. 2 is used for the vitrification of the glass preform in the form of a rod under the reduced or the vacuumed atmosphere and a predetermined temperature condition as schematically shown in Fig. 3 (a vitrification temperature is usually in a range of 1550 to 1650° C) in the conventional manner. The produced glass preform has a longitudinally non-uniform shape in its outer diameter as schematically shown in Fig. 4 which shows both end portions of the preform thicker and the central portion thereof thinner. In order to produce a glass article having a good quality from the glass rod produced by the above process, existence of bubbles should be suppressed or eliminated. In addition, the outer diameter of the vitrified glass rod should be uniform. EP-A-0 523 692 discloses a method for producing a glass preform for an optical fiber. The method comprises heating a glass soot preform to remove gas at a temperature at which the soot preform is not vitrified and under reduced pressure. The preform is then further heated under reduced pressure to the vitrification temperature. During the first heating step, the temperature may be increased stepwise punctuated by pauses at interim temperatures. SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved process for the production of a glass preform for an optical fiber which includes substantially no bubbles therein and which has a substantially uniform shape so as to overcome the above problems. According to the present invention, there is provided a process for the production of a silica-based glass preform for an optical fiber comprising: (i) depositing fine glass particles on a rod to form a body, (ii) heating the body under reduced pressure at a first temperature to degas it, (iii) heating the degassed body under reduced pressure at a second temperature which is higher than the first temperature to shrink it, and (iv) heating the shrunken body under reduced pressure at a third temperature which is higher than the second temperature and which causes vitrification of the fine glass particles to produce the glass preform; characterised in that the heating step (iii) for shrinking the body is carried out in such a way that different portions of the body in its longitudinal direction are heated to different second temperatures which are controllable independently of each other, the body being oriented to have a lower and an upper portion, the temperature of the lower portion being greater than the temperature of the upper portion. In the present invention, the fine glass particle deposited body may be produced by any conventional process such as the VAD process or the OVD process. In principle, the atmosphere during the heating steps is continuously or successively evacuated in a conventional means such as a vacuum pump from an apparatus in which the heating steps are carried out. Optionally, the atmosphere may comprise an inert gas, a halogen gas, a halogenated gas or a mixture thereof for a predetermined purpose such as dehydration, fluorine doping and the like, if necessary, according to the conventional knowledge, provided that the following specified pressure (degree of vacuum) and the following specified heating temperature are ensured. For example, the body may be a composite which is produced by depositing the fine glass particles around a glass rod having an at least double waveguide structure of which refractive index is lower in its outer portion than its core portion. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 schematically shows a heating furnace used in one embodiment of the present process in which a plurality of (for example three) heating elements (3, 4 and 5) are provided so as to constitute a heating member, in which the numeral (1) indicates a fine glass particle deposited body, (2) does a muffle tube in which the body (1) is inserted and heated, (3) does an upper heating element, (4) does a middle heating element, (5) does a lower heating element, (6) does a degassing conduit, (7) does a vacuum pump, (9) does a heat shield and (10) does a vacuum vessel; Fig. 2 schematically shows a conventional heating furnace; Fig. 3 is a graph which schematically shows a temperature condition in a conventional heating process to have a vitrified glass preform; Fig. 4 schematically shows a shape of a produced glass preform in a conventional process which has different diameters along its length; Fig. 5 is a graph showing a relationship between a first heating temperature and an attained degree of vacuum (namely an attained reduced pressure) in a first heating step and also a relationship between the first heating temperature and a number of bubbles contained in a vitrified glass preform per a unit length of the preform; Fig. 6 is a graph showing a relationship between a retention time at a second heating temperature and an outer diameter difference of, a vitrified glass preform; Fig. 7 is a graph showing a relationship between a second heating temperature and an outer diameter of a vitrified glass preform; and Fig. 8 is a graph showing a relationship of a temperature difference between a lower heating element and an upper heating element versus an outer diameter difference of a vitrified preform between around a lower portion and around a middle portion of the preform. DETAILED DESCRIPTION OF THE INVENTION According to the present invention, the first heating step is preferably continued in a temperature range of 1000 to 1300° C while the atmosphere in the furnace is evacuated with a vacuum pump until a pressure of the atmosphere (a degree of vacuum) reaches 10 Pa or less and more preferably 5 Pa or less. The second heating step is preferably carried out at a pressure of 10 Pa or less and more preferably 5 Pa or less with keeping the second heating temperature in a range of 1320 to 1480° C and more preferably 1350 to 1450° C for 10 to 120 minutes and more preferably 40 to 90 minutes while the evacuation is continued if necessary. In addition, a heating member which is located around a muffle tube and which heats the degassed body may comprise a plurality of heating elements which are located separately along a longitudinal direction of the body and which can control the heating temperature of degassed body portions independently from one another so that the heating elements can heat the degassed body portions to different temperatures along the length of the preform. Also, the plurality of the heating elements may be so arranged that a lower portion of the degassed body is heated to a temperature equal to or higher than a temperature of an upper portion of the degassed body which is located above the lower portion. The third heating step is carried out at a pressure of 10 Pa or less and more preferably 5 Pa or less in a temperature range of 1490 to 1600° C and more preferably 1500 to 1550° C for 1 to 60 minutes and more preferably 5 to 20 minutes. The present inventors carried out the heating treatment of a fine glass particle deposited body in a reduced atmosphere while evacuated with the vacuum pump, and obtained a relationship between the first heating temperature and an attained degree of vacuum. A solid line in the graph of Fig. 5 shows such a relationship after 5 hours have been passed from the beginning of the heating. As seen from the graph, when the first heating temperature is 1000° C or more, the attained degree of vacuum is 10 Pa or less. After the first heating step, the body was heated to 1550° C so as to vitrify the body and then a number of bubbles contained in the vitrified body was obtained. This illustrates the effect of some features of the present invention. The results are also shown in the graph of Fig. 5 with a broken line, which shows a relationship between the first heating temperature and the number of the bubbles finally confined in the vitrified body per one meter length of the body. As seen from the broken line, when the first heating temperature is in a range of 1000 to 1300° C, nearly no bubble is left in the body. On the other hand, when the first temperature is in the outside of such a specified range, the number of the contained bubbles is sharply increased. Although not bound by any theory, the reason why the above results were obtained could be as follows: A degassing rate itself from the fine glass particle deposited body is larger at a higher first heating temperature. However, the body shrinks more readily at the higher degassing temperature so that gases are not tend to be removed due to the shrinkage. Thus, since the degassing rate is smaller at the first heating temperature below 1000° C or since the shrinkage already begins before a sufficient amount of the gas has been removed at the first heating temperature above 1300° C, the bubbles are likely to be confined in the body. Therefore, on the basis of the results shown in Fig. 5, it is critical to produce the vitrified glass preform with substantially no bubble that the glass body is heated to a temperature range of 1000 to 1300° C until the pressure inside the furnace reaches 10 Pa or less, and then the body is further heated to the higher temperature so as to vitrify the body. In the production of the optical fiber preform, one more factor which is as important as the number of the bubbles contained in the preform is uniformity of an outer diameter of the glass preform along its length. The present inventors have found that there is a strong correlation of a retention time during which the degassed body is keep at the second heating temperature in a range of 1320 to 1480° C versus an outer diameter difference between around a middle portion and around a lower portion of the body (namely, "a diameter around the lower portion of the body" minus "a diameter around the central portion of the body") in the finally produced preform. Such a correlation is shown in Fig. 6. In addition, the inventors have further found a correlation between the second heating temperature and an outer diameter of a middle portion of the glass preform body after finally vitrified, and the correlation is shown in Fig. 7. This is because shrinkage along a radial direction of the body is larger at a higher temperature so that such a correlation is obtained. On the basis of the results shown in Fig. 7, it is effective to carry out the second heating step so as to increase the temperature of the lower portion of the degassed body relative to the temperature of the upper portion of the degassed body and/or to decrease the temperature of the upper portion of the degassed body relative to the temperature of the lower portion of the degassed body in order to prevent the larger outer diameter of the lower portion of the glass body. Therefore, one preferred embodiment according to the present invention may use a heating furnace as shown in Fig. 1 in which a heating member is divided into a plurality of heating elements (for example three heating elements 3, 4 and 5) located along a longitudinal direction of the degassed body and the heating temperature of each heating element is controlled independently from one another, so that the lower portion of the body may be heated to a temperature higher than a temperature to which the upper potion is heated. Using the heating furnace as shown in Fig. 1, a relationship, as shown in Fig. 8, was obtained as to the heating temperature difference between the lower heating element and the upper heating element versus the outer diameter difference between around the lower portion and the middle portion of the body. It has been found from the results that the diameter difference becomes less than 6 mm when the temperature of the lower heating element is higher than that of the upper heating element by 50° C or more. The third heating step is carried out in a range of 1490 to 1600° C which corresponds to the vitrification temperature of the glass particle deposited body. It has been found that a longer period of the third heating step than as required causes droop of the vitrified preform due to its weight since the glass is very soft at the third heating temperature. Therefore, the period of the third heating step should be as short as required, and thus the period is preferably in a range of 1 to 60 minutes and more preferably 5 to 20 minutes. As described above, since the fine glass particle deposited body is heated under such conditions that the gas contained in the body is sufficiently removed according to the present invention, substantially no bubble is left in the vitrified glass body. In addition, the vitrified glass body having the uniform outer diameter is produced by relatively raising the heating temperature of the lower portion of the fine glass particle deposited body with using the heating member which is divided into the plurality of the independent heating elements. EXAMPLES The present invention will be, hereinafter, described with reference to the following Examples and Comparative Example. It should be understood that the present invention is not limited to the Examples. Example 1 Using the heating furnace as shown in Fig. 1, a fine silica glass particle deposited body (1) produced by the VAD process was heated to be vitrified according to the present invention. The body had outer diameter of 200 mm and a length of 1000 mm. The body was inserted in a muffle tube (2) located in the vacuum vessel (10) and heated to a temperature of 1200° C while the atmosphere was evacuated from the furnace. When the temperature was reached to 1200° C, the pressure in the furnace was 20 Pa. However, after the furnace was kept at that temperature for three hours, the pressure was reduced to 3 Pa. Then, a temperature of the upper heating element 3 was increased to 1330° C, a temperature of the middle heating element 4 was increased to 1400° C, and a temperature of the lower heater was increased to 1470° C. Such an increased temperature condition was kept for one hour, and the pressure inside the furnace was constant at about 3 Pa during said one hour. Thereafter, the body was heated to 1550° C and kept at that temperature for 15 minutes, and the pressure inside the furnace was still constant at about 3 Pa. Then the body was cooled and observed. The body was of a so good quality that it had substantially no bubble over its whole length. The outer diameter of the body was substantially uniform such that it is 90 mm ± 1.2 mm over its whole length. Comparative Example 1 The same fine glass particle deposited body as in Example 1 was inserted in a muffle tube 2 placed in the vacuum vessel as shown in Fig. 2, and the pressure inside the furnace was kept at 30 Pa and heated to a temperature of 1600° C for 30 minutes. Then, the body was cooled and taken out of the furnace. The body contained small bubbles having a size of 0.1 mm or less over its whole length and having a deformed form such that an outer diameter around the center portion was so thin as 76 mm, and the outer diameter around a lower portion was so thick as 90 mm. Example 2 The same fine glass particle deposited body as in Example 1 was inserted in the muffle tube 2 placed in the apparatus as shown in Fig. 1 which had been already preheated to a temperature of 800° C. When the body was heated to 1300° C and kept at such a temperature for two hours while evacuated in the first heating step, the pressure inside the furnace attained to 5 Pa. Then, the body is further heated so as to initiate the second heating step in which the upper heating element was set to 1320° C, the middle one to 1390° C and the lower one to 1470° C and such a different temperature condition was kept for one hour. Thereafter, in order to carry out the third heating step, the upper heating element was set to 1550° C, the middle one to 1525° C and the lower one to 1500° C and such a different increased temperature condition was kept for 30 minutes, and then cooled. During the third heating step, the pressure inside the furnace was constant at 5 Pa. The produced glass preform was of a so good quality that it had substantially no bubble over its whole length. The outer diameter of the body was substantially uniform such that it is 90 mm ± 0.8 mm over its whole length.

Claims: A process for the production of a silica-based glass preform for an optical fiber comprising: (i) depositing fine glass particles on a rod to form a body (1), (ii) heating the body under reduced pressure at a first temperature to degas it, (iii) heating the degassed body under reduced pressure at a second temperature which is higher than the first temperature to shrink it, and (iv) heating the shrunken body under reduced pressure at a third temperature which is higher than the second temperature and which causes vitrification of the fine glass particles to produce the glass preform; characterised in that the heating step (iii) for shrinking the body is carried out in such a way that different portions of the body in its longitudinal direction are heated to different second temperatures which are controllable independently of each other, the body being oriented to have a lower and an upper portion, the temperature of the lower portion being greater than the temperature of the upper portion. A process according to Claim 1, wherein the first temperature is in the range 1000-1300°C, and the heating step (ii) is carried out while lowering the pressure until the pressure reaches 10 Pa or less. A process according to Claim 1 or Claim 2, wherein the heating step (iii) for shrinking the body comprises heating the body at a temperature in the range 1320-1480°C at a pressure of 10 Pa or less for 10-120 minutes. A process according to any preceding Claim, wherein the heating step (iv) for vitrifying the fine glass particles comprises heating the body at a temperature in the range 1490-1600°C at a pressure of 10 Pa or less for 1-60 minutes. A process according to any preceding Claim, wherein the rod is a glass rod having an at least double waveguide structure whose periphery has a lower refractive index than its core.

Inventors: DANZUKA TOSHIO, HOSHINO SUMIO, ITO MASUMI, OHGA YUICHI, TSUCHIYA ICHIRO, DANZUKA, TOSHIO, HOSHINO, SUMIO, ITO, MASUMI, OHGA, YUICHI, TSUCHIYA, ICHIRO, DANZUKA, TOSHIO, C/O YOKOHAMA WORKS OF, HOSHINO, SUMIO, C/O YOKOHAMA WORKS OF, ITO, MASUMI, C/O YOKOHAMA WORKS OF, OHGA, YUICHI, C/O YOKOHAMA WORKS OF, TSUCHIYA, ICHIRO, C/O YOKOHAMA WORKS OF
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD
Application year: 1994
Main classification: C03B37
IPC codes: C03B37, C03B8, C03B20, G02B6